The invention relates to a circuit arrangement for obtaining a phase-sensitive control quantity which depends on the phase shift of a color burst signal having an amplitude factor, with respect to a reference signal, preferably of approximately the same frequency, which is phase-locked to a clock signal with which the amplitude values of the color burst signal are sampled and whose frequency is the 4n-fold of the frequency of the reference signal,
wherein n is an integer of the series 1, 2, . . . and the keying occurs in such manner that a first sampled difference signal (X, for example A.multidot.(sin .alpha.+cos .alpha.)) and a second sampled difference signal (Y, for example A.multidot.(cos .alpha.-sin .alpha.)) shifted by a quarter of a cycle of the color burst signal are obtained, wherein said sampled difference signals are proportional to the amplitude (A) of the color burst signal and which depend on the phase angle (.alpha.) of the color burst signal with respect to a point of time determined by the reference signal according to an angular function, said sampled difference signal (Y) being the difference between a third and a first of said samples while said sampled difference signal (X) being the difference between a fourth and a second of said samples,
in which the sampled difference signals (X and Y) are squared and the root is extracted from their sum in such manner that the amplitude factor is obtained
and in which furthermore, by means of at least one conversion stage to the input of which the first and the second signal samples are applied, at least one phase-sensitive control quantity, which depends on the phase angle (.alpha.) and is at least substantially independent of the amplitude factor, is obtained.
Such a circuit arrangement is known from the European Patent Application No. 00 37 317 which corresponds to U.S. Pat. No. 4,412,181. In this arrangement, signal samples which correspond to A.multidot.sin .alpha. and A.multidot.cos .alpha., are obtained from the chrominance signal and the color burst signal by sampling. From these the angle .alpha. should be obtained in an amplitude-independent manner, in that the tangent is formed by the quotient of both values and then the angle (inverse tangent) is derived. During the color burst signal, an angle of deviation, if any, if so determined and by addition and subtraction, respectively, to the angle of the color vector is eliminated there. Finally, the value of the color vector obtained separately from said angle determination is multiplied by the sin and cos, respectively, of the corrected color angle and the pure components of the chrominance signal are so obtained.
The angle obtained via the tangent, however, is uncertain by 180.degree. since the tangents in the first and second and in the third and fourth quadrants, respectively, are equal according to amount and slope. Moreover, the tangent goes to infinity so that a very large scope of values is to be computed and processed.